Scientists studying the massive earthquake that struck the South Pacific on September 29, 2009, have found that it actually involved two great earthquakes: an initial one with magnitude 8.1, which then triggered another magnitude 8 earthquake seconds later on a different fault. The details of this rare event, called a "triggered doublet," are unlike anything seismologists have seen before.

"We know of no precedent for the Samoa triggered doublet," said Thorne Lay, professor of Earth and planetary sciences at the University of California, Santa Cruz, who led a seismological analysis of the event published in the August 19 issue of Nature.

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The earthquakes unleashed devastating tsunami waves that swept onto the islands of Samoa, American Samoa, and Tonga, killing 192 people. It took months, however, for seismologists to make sense of the confusing seismic data and figure out exactly what happened in the Earth's crust to cause this disaster.

Most great earthquakes (earthquakes of magnitude 8 or greater) occur in subduction zones, where one plate of the Earth's crust dives beneath another plate. The Tonga subduction zone in the South Pacific marks the boundary where the Pacific plate is sinking under the Australian plate.

In the sequence of events on September 29, the first earthquake actually occurred not at the subduction zone, but within the Pacific plate at a site 50 to 100 kilometers (30 to 60 miles) east of the plate boundary. The rupture occurred along an extensional or "pull-apart" fault in the middle of the plate. Such large extensional faulting near a subduction zone is rare, and this is the third largest such event recorded in the 110-year history of seismological monitoring.

According to Lay, these events seem to occur in areas where the plate boundary is relatively weak and one plate slides easily under the other. As the leading edge of the oceanic plate sinks into the mantle, it pulls on the rest of the plate, bending it downward and causing it to break along the extensional fault.

"We think the subducted slab is pulling on the Pacific plate, and that 'slab pull' is responsible for the bending and extension of the plate manifested in the Samoa earthquake," he said.

As seismologists in several programs, including Lay's team, began to study the event in more detail, they noticed some strange inconsistencies. Aftershocks were spread over a huge area, including the Tonga subduction zone, with relatively few along the fault in the Pacific plate that had ruptured. In addition, when researchers used different methods to calculate the fault geometry, they came up with inconsistent solutions, which is rare for large earthquakes. Finally, tsunami prediction models gave results that were not entirely consistent with observations from ocean buoys and on the islands.